1. Field of the Invention
The present invention relates to a catalyst for cleaving alkyl tert-alkyl ethers or tertiary alcohols to isoolefins and alcohol or water.
2. Description of the Related Art
Isoolefins, for example isobutene, are important intermediates for the preparation of a multitude of organic compounds. Isobutene is, for example, a starting material for the preparation of butyl rubber, polyisobutylene, isobutene oligomers, branched C5 aldehydes, C5 carboxylic acids, C5 alcohols and C5 olefins. It is also used as an alkylating agent, especially for the synthesis of tert-butylaromatics, and as an intermediate for obtaining peroxides. In addition, isobutene can be used as a precursor for methacrylic acid and its esters.
In industrial streams, isoolefins are usually present together with other olefins and saturated hydrocarbons with the same number of carbon atoms. The isoolefins cannot be removed in an economically viable manner from these mixtures with physical separating methods alone.
For example, isobutene is present in typical industrial streams together with saturated and unsaturated C4 hydrocarbons. Owing to the small boiling point difference and the small separating factor between isobutene and 1-butene, isobutene cannot be removed from these mixtures in an economically viable manner by distillation. Isobutene is therefore frequently obtained from industrial hydrocarbons by converting isobutene to a derivative which can be removed easily from the remaining hydrocarbon mixture, and by dissociating the isolated derivative back to isobutene and derivatizing agent.
Typically, isobutene is removed from C4 cuts, for example the C4 fraction of a steamcracker, as follows. After removal of the majority of the polyunsaturated hydrocarbons, mainly the butadiene, by extraction/extractive distillation or selective hydrogenation to give linear butenes, the remaining mixture (raffinate I or selectively hydrogenated crack-C4) is reacted with alcohol or water. Isobutene forms methyl tert-butyl ether (MTBE) when methanol is used, ethyl tert-butyl ether (ETBE) when ethanol is used and tert-butanol (TBA) when water is used. After they have been removed, these derivatives can be cleaved to isobutene in a reversal of their formation.
The cleavage of alkyl tert-butyl ethers (ATBE) to the corresponding isoolefins and alcohols and the cleavage of tertiary alcohols to the corresponding isoolefins and water can be performed in the presence of acidic or basic catalysts in the liquid phase or gas/liquid mixed phase or in the pure gas phase.
The cleavage in the liquid phase or gas/liquid phase has the disadvantage that the products formed, dissolved in the liquid phase, can enter into side reactions more easily. For example, the isobutene formed in the cleavage of MTBE can form undesired C8 and C12 components as a result of acid-catalysed dimerization or oligomerization. The undesired C8 components are mainly 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. In addition, some of the methanol formed in the cleavage is converted to dimethyl ether with elimination of water particularly over basic catalysts. When the reaction is not performed under pressures above the saturation vapour pressure of the reaction mixture, in order to counteract these problems, an additional solvent is necessary.
In the gas phase, the formation of by-products as a result of reaction of the cleavage products with themselves can be suppressed owing to their lower concentrations in comparison to the cleavage in the liquid phase. However, other side reactions can occur owing to the relatively high cleavage temperatures. In the gas phase cleavage, catalysts are therefore required which catalyse the cleavage of tertiary alkyl ethers or tertiary alcohols to isoolefin and alcohol or water with very high selectivity, but do not promote any side reactions, for example C—C cleavage or dehydrogenation and C—C coupling reactions or ether formation of the alcohols formed. Moreover, these catalysts should enable high space-time yields and have a long lifetime. In addition, cleavage of the reactant with maximum selectivity for the isoolefin formed at a pressure of greater than 0.3 MPa is desirable.
The catalysts described in the literature for the gas phase cleavage of alkyl tert-alkyl ethers (ATAE) and tertiary alcohols to the corresponding isoolefins and alcohol or water are a multitude of catalysts. This is true in particular for catalysts which are utilized for the cleavage of methyl tert-butyl ether (MTBE).
The catalysts used are usually metal oxides having an empirical formula of MaOx, mixed metal oxides with empirical formulae MaMbMnOy, especially those which contain M=Si or M=Al, acids on metal oxide supports or metal salts.
U.S. Pat. No. 4,254,290 describes, as cleavage catalysts, for example, SiO2/Al2O3 or WO3/Al2O3. U.S. Pat. No. 4,320,232 and U.S. Pat. No. 4,521,638 claim, for the cleavage of tertiary ethers, catalysts consisting of phosphoric acid on supports. Aluminum oxide on silica gel is utilized as a cleavage catalyst in U.S. Pat. No. 4,398,051. In the two patents U.S. Pat. No. 4,357,147 and U.S. Pat. No. 5,254,785, zeolites are used for the same purpose.
In JP 59010528, the cleavage catalyst used is sulphated titanium dioxide or zirconium dioxide. Ethers are cleaved in U.S. Pat. No. 5,607,992 by using a zirconium oxide/cerium oxide catalyst, in U.S. Pat. No. 6,124,232 by using zirconium oxide/tungsten oxide, in U.S. Pat. No. 6,162,757 by using a mixed oxide of zirconium and rare earths.
WO 2005-066101 claims a catalyst with the general empirical formula XmYnZpOq where X is at least one element of the fourth group of the Periodic Table of the Elements, Y is at least one metal from the third and/or sixth group and Z is at least one element from the seventh, eighth or eleventh group.
JP 1993-229965 claims a catalyst with the empirical formula SiaXbYcZdOe. (Here, Si and O in each case are silicon and oxygen; X is at least one element which is selected from the group consisting of titanium and zirconium; Y is an element which is selected from the group consisting of magnesium and calcium; Z is at least one element which is selected from the group consisting of sodium, potassium, chlorine and sulphur; a, b, c, d and e indicate the atomic ratio of the individual elements. When a=1, b=0.001 to 10, c=0.0001 to 5, d=0 to 1; e is the number of oxygen atoms needed to satisfy the valency of the individual constituents mentioned above.)
U.S. Pat. No. 5,171,920 describes, in Example 4, the preparation of a cleavage catalyst which, in a formal sense, contains the components silicon dioxide, aluminum oxide and magnesium oxide. The preparation is done in such a way that silicon dioxide is first saturated/impregnated with an aqueous magnesium nitrate solution, and an intermediate drying is followed by a further saturation/impregnation with an aqueous aluminum nitrate solution. Subsequently, predrying is followed by calcination.
EP 0 589 557 claims, inter alia, a cleavage catalyst which consists, in a formal sense, of magnesium oxide, aluminum oxide and silicon dioxide. In its preparation, an aluminosilicate is impregnated in a first step with an aqueous magnesium salt solution in such a way that, during the impregnation, the pH of the impregnation solution can be adjusted to a pH of 7 to 11 by adding a base. In order to obtain particularly active and selective catalysts, impregnation times of over 200 h are required in some cases.
In the cleavage of alkyl tert-alkyl ethers or tertiary alcohols to isoolefin and alcohol, the known catalysts have one or more of the following disadvantages: low selectivity for the target products, use of high temperatures in the cleavage, in some cases above 500° C., short lifetimes of the catalysts, and complicated and hence costly preparation of the catalyst.